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In atomic physics, it is common knowledge that following the absorption edge, where the photon energy equals the binding energy of a core electron, a monotonic decrease in the absorption coefficient with increasing photon energy is observed. Obviously it is so common, that everyone mentions it, but nobody cares to explain. For example:

absorbtion spectra graph

As I understand, in EXAFS (Extended X-Ray Absorption Fine Structure) case, a core electron is excited to the conduction band, and photons with higher energy should just excite it to the higher energy level in the conduction band. So why does absorption decrease?

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Far above the resonance, you can apply a classical oscillator model to estimate the absorption of an electromagnetic wave. This happens to have a dependence on $1/\omega^3$ or $1/E^3$, sometimes called Kramer's formula (that one is actually a quantum derivation). This matches the decrease observed after the peak.

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